Acoustic device



NOV- 12, 940 L. A. MORRISON ET Al.A 2,220,942

ACOUSTIC DEVCE Filed Aug. l, 15956 3 Sheets-Sheet l I 20 i Y 2a @y 1 2/ W ATTO/WEL Nov. 12,1940.

POLE PIECE AREr-SOUARE INCHES L A. MQRRlsoN ETAL 2,220,942

ACOUSTIC `DEVI CE v Filed Aug. 1, 195e l 5 Sheets-Sheet 2,

FREQUENCE-KM OC3/CLES PER SEC.

- L. ,4. MORRISON /NVENro/a?. 5 E MOTT Nov. 12, 1940. L. A. MORRISON ETAL 2,220,942

ACOUSTIC DEVICE i Filed Aug. l, 1936 5 Sheets-Sheet 3 v nqooo V REACTANCE-OHMS L. A. MORR/S E. E. MO T 7 A TTORNEV and secured thereto by screws 29. Terminal plate 30 of insulating material extending across the rear of the assembly is also secured in position by screws 29. The ends of the pole-pieces pass between the magnets and project into the central aperture of the foundation frame. Speech current coils 3| are mounted on the pole-pieces and have their ends brought out to concentric contact elements 32 and 33 mounted on plate 30.

The front face of the foundation frame 25 is provided with an annular ridge 34 surrounding y a central recessed portion. A fiat circular diaphragm 35 of magnetic material is supported at its periphery by the ridge, and is held in position by the attraction of the pole-pieces. The diaphragm is not mechanically clamped and is therefore free to expand and contract in response to temperature changes without being subjected to mechanical stresses. The form of the supporting frame 25 and the method of supporting the magnetic elements therefrom facilitate accurate machining and assembly .of the parts and ensure the establishment and maintenance of a precise separation between the diaphragm and the faces of the pole-pieces.

A damping plate 36 of insulating material, provided with suitable close fitting apertures through which pole-pieces 28 project, is inserted in the central recessed portion of frame 25 to which it may be riveted, cemented, bonded or otherwise secured. The damping effect is obtained by the use of' a plurality of very narrow air leakage paths leading through plate 36 from the air chamber 38 between the plate and the diaphragm. In the present structure, these are provided by an aperture 40 in the damping plate over which is laid one or more strips of silk fabric 50. Other known arrangements for this purpose may also be used, such as any fibrous or porous material through which the air may fiow against frictional forces. The front of the diaphragm is enclosed by a recessed cover plate 4I of insulating material, which is secured to frame 25 by rivets 42. 'I'he recessed portion of the cover plate is large enough in diameter to clear the diaphragm and is deep enough to provide an air chamber 43 of proper dimensions in front thereof. A group of small, centrally located apertures 44 provide sound wave outlets from air chambers 43.

A typical magnet structure would comprise parts approximately of the following compositions and dimensions: permanent magnets of 35 per cent cobalt steel, 1.25 inches long, and onesixteenth of a square inch in cross-sectional area; pole-pieces of a nickel-iron alloy in the approximate proportions 45:55, each having a cross-sectional area of .031 square inch; a diaphragm of cobalt iron-vanadium alloy in the approximate proportions 49:49:2, its thickness being .011 inch, its diameter 1.46 inches and its normal air-gap from the pole faces substantially six-thousandths of an inch.

The receiver illustrated in Fig. 'l is. in general, similar to that of the preceding figures, but differs therefrom in certain features of the mechanical structure and in the arrangement of the acoustic damping elements. The foundation frame 45 is of insulating material instead of metal, and may consist of a molded phenol plastic part. It is similar in shape to the frame 25 of the preceding example, but includes the damping plate back of the diaphragm as an integral element with apertures molded to receive the magnet pole-pieces. The magnet system and the diaphragm are of the same construction, and are supported in the manner already described. The complete assembly unit is adapted for mounting in the same way as that of Figs. l to 6, and may be interchangeable therewith. In the figure, it is shown mounted in a handset telephone.

For the purpose of damping, an aperture 46 in the front of frame 45 communicates with a chamber 41 formed in the rear of the frame. A disc 46 positioned across the rear of aperture 46 and provided with a plurality of fine h oles or slots provides the desired acoustic resistance. Chamber 41 is closed by a back plate 49, cemented in place, thereby limiting the motion of the air through the acoustic resistance and modifying the action thereof. In an experimental model of this receiver, the magnetic structure was the same as that of the previous example except that a diaphragm of slightly larger diameter, 1.565 inches, Was used.

The invention resides, in part, in the coordination of the structural proportions of the mechanicalelements with the magnetic characteristics of the materials of the system and with the acoustic elements of the system. The rnechanical structures illustrated in Figs. l to '1 are of importance in that they embody features whereby the principles of the coordination may be applied with a high degree of accuracy, and whereby the stability of the improved operating characteristics is ensured. The manner in which the proportions of the system are coordinated will be more readily understood and appreciated from the following considerations.

The function of a telephone receiver of the type described is to produce a pressure variation in the cavityof the ear to which it is applied, which follows as faithfully as possible the variations of the speech' current applied to the device. Since the ear cavity forms a substantially closed air space directly coupled to the diaphragm of the receiver, the pressure variation will be dependent on the amplitude of the diaphragm motion rather than on its velocity. While the air space enclosed by the application of a receiver to the ear is, in general, not completely closet. we have found that the effect of the air leakage paths is large only at quite low frequencies, and is barely noticeable at frequencies above 200 or 300 cycles per second. It is assumed, of course, that the instrument is held fairly firmly against the ear.

For the purpose of examining the response characteristic of any given receiver, we have found that it is permissible to replace the ear cavity by a rigid-walled chamber of six cubic centimeters volume coupled directly to the outlet aperture in the receiver cap and having no outlet to the external air. The pressure variation measured in this cavity is a reliable measure of the sensitivity of the receiver and furnishes an accurate basis for the comparison of different designs. The use of rigid walls in the chamber is justified by the fact that the energy absorbed by the walls of the ear cavity is negligibly small compared with the energy dissipated elsewhere in the receiver system. The volume six cubic centimeters, has been found by many measurements to be a good approximation to the normal 70 human ear, and is, therefore, representative of the conditions under which the receiver will most frequently operate.

The system comprising the receiver and the closed chamber load is one which lends itself to 75 analysis both experimentally and mathematically. For this purpose, the receiver shown in Figs. 1 to 6 may be represented schematically by the diagram of Fig. 8. Here T1 and T2 are the input terminals of the receiver, to which are connected an oscillation source of voltage E and internal :resistance R. 'Ihe receiver winding impedance, measured with the diaphragm held fast, is represented by resistances r1 and r2 and inductances L1 and L2. Resistance r1 is the direct current resistance of the Winding, inductance L1 is the inductance of that part of the winding which is substantially uncoupled to the magnetic clrcuit, inductance L2 is the inductance of the cou pled, or effective, part of the winding, and resistance r2 represents the eddy current loss in the magnetic elements. The electrical part of the circuit is coupled to the mechanical-acous- Itical part by virtue of the -force factor which is indicated schematically by the element The elements of the mechanical-acoustical part of the system are indicated in the diagram as follows: M0 denotes the effective `mass of the diaphragm, So its effective flexural stiffness, and Ro the mechanical resistance due to hysteresis. The stiffness of the air chamber 38 back of the diaphragm is represented by S1 and the acoustic impedance of the leakage path through damping element 40 by resistance R1 and mass M1. S2 denotes the stiffness of the air chamber 43 in front of the diaphragm, R3 and M3 are respectively the resistance and mass of the combined outlet apertures 44, and S3 is the stiffness of the enclosed ear cavity which, for the reasons indicated, is taken as that of an air volume six cubic centimeters connected to apertures 44. Resistances Ro and R3 are sufficiently small in ccmpariscn with the other resistances of the system so that they do not materially affect the re- 40 sponse.

At very `low frequencies, Where the reactances of the mass and inductance elements are negligibly small, it may be shown -that the amplitude of the displacement of the diaphragm is given by wherein a denotes the amplitude, CLthe magnitude of the force factor, and .I the amplitude of vthe current in the electrical circuit, all values being in c. g. s. units. S1 may be omitted because it is negligibly small. If the masses of the system could be eliminated, no damping would be necessary, since, in the absence of mechanical resistance, the fixed proportionality between the current and the displacement expressed by Equation 1 Would then hold at all frequencies.

Because of the frnass of the diaphragm, the amplitude of the displacement varies with frequency and exhibits a maximum aft a resonance frequency determined substantially by the mass M0 and the combined stiffnesses Se, S2 and S3. By proper proportioning of the damping, the amplitude at this resonance may be made equal to the low frequency value given by Equation l. At frequencies below resonance, the displacement then remains substantially constant, but at highn er frequencies it tends to fall olf very rapidly. The resonance frequency of the diaphragm is, therefore, a measure of the frequency range through which uniform response can be maintained. An extension of the frequency range above the diaphragm resonance is obtained by proportioning the outlet apertures 44, which conu` ple the front of the diaphragm with the ear cavity, so that the mass of air therein resonates with lthe stiffness of the ear cavity and the front air chamber 43 ai; a higher frequency. This resoM nant system corresponds to the loop S2, S3, Ms, R3, in Fig. 8. However, to maintain uniformity of the response over the extended range, the above noted ear cavity resonance frequency must bear a substantially fixed proportion to the resonance frequency of the diaphragm, which proportion we have found fto be substantially equal to 1:2. The utility of the diaphragm resonance frequency as a measure of the operating range is, therefore, not affected by modifications of the acoustic system.

The diaphragm resonance frequency is given by the relationship,

Q' l @eeuws-2a) the left-hand side of which represents the product of the displacement perumt currentmultiplied by the square of the frequency range, and the right-hand side of which is a constantl for a given structure. Since the performance of a receiver is gauged by the combination of the two factors, sensitivity and frequency range, it will be seen from Equation 3 that a high level of performance depends not on the magnitude of the force factor alone, but on the ratio of this quantity to the effective mass of the diaphragm.

In Equations l and 3, the response is indicated by the amplitude of the diaphragm displacement. More correctly, it should be represented by the pressure increment in the ear cavity represented by the stiffness S3. At low frequencies, and hence at all frequencies in the response range of a properly damped device, the increment of pressure due to the diaphragm displacement is given by rum-vi (t) where p denotes the pressure increment in the ear cavity, P the normal atmospheric pressure, *y the adiabatic constant for air, A the effective diaphragm area, and V2 and Vs respectively the volumes of the air chamber 43 in front of the diaphragm and of the normal ear cavity. By means of Equations 3 and e, a modified performance equation is obtained, namely, f

p psy GA iwf-vain "in (t) in which the sensitivity of the device appears as lthe pressure increment in the ear cavity per unit of current in the receiver windings. The quantities P and y are physical constants and, since the volume V2 will generallybe small compared with Vs, the sum of the two 'volumes may be taken as substantially equal to V3, which may also be considered to be a physical constant. The ratio Pfy/ (Vz-l-Vs) is, therefore, a constant which is independent of the receiver design.

It may, further, be shown that the force factor G, which is defined as the ratio of the mechanical force produced to the magnitude of the curm rent producing it, may be expressed in terms of the magnetic fiux and the number of turns in the receiver winding by the equation,

in which the sensitivity is expressed as the more basic ratio ofthe acoustic pressure to the ampera turns of the winding. The factor Go appearing on the right-hand side is likewise a basic parameter of the magnetic structure since it depends wholly on the geometry of the structure and the magnetic properties of the materials. The expression,

AG1) E (8) may, therefore, be regarded as a fundamental gure of merit by means of which the performances of diiferent types of telephone receivers may be compared. In the case of receivers, such as those of the invention, in which magnetic diaphragms of uniform thickness are used, the ratio of the effective area. to the effective mass is substantially inversely proportional to the diaphragm thickness. the densities of the various ferromagnetic materials being approximately alike. The figure of merit is therefore proportional to the ratio of the force factor to the diaphragm thick- 0 ness.

To secure a high figure of merit, it is desirable that the diaphragm have a large effective area and a small effective mass, and that the flux traversing the magnetic circuit vary rapidly with the diaphragm displacement. 'I'he value of theflgure of merit is dependent on the length of the airgap, the cross-sectional area of the pole-pieces and the thickness of the diaphragm, all of which may be varied independently. As the air-gap is decreased, the gure of merit increases continuously, but practical considerations of manufacture and of stability of operation limit the minimum length of air-gap, and hence, also, 'the improvement in the figure of merit that can be obtained inthis way.

From. an extensive series of measurements of the force factor in structures employing electromagnets as the source of polarizing flux we have found that, for a given set of structural dimensions, air-gap, pole face area and diaphragm thickness, the figure of merit exhibits a maximum for a particular value of the polar magnetizing force. Further, when the diaphragm thickness is varied independently, the magnetizing force being adjusted in each case to the op- We have also'found that the gure of merit reaches a grand maximum for a particular combination of these dimensions. 'I'his represents an absolute maximum of the figure obtainable with a given air-gap and given materials in the pole-pieces and diaphragm. To achieve the maximum there is required a proper coordination of the three quantities, polarizing flu'x, pole face area, and diaphragm thickness. Coordination of the pole face area and the diaphragm thickness without regard to the polarizing flux will result in a partial maximum with respect to variations oi' these quantities. but to obtain the absolute maximum it is necessary that the polarizing flux also have a value for optimum effect.

In the experimental work referred to, the rate of change of the magnetic iiux was measured directly by giving the diaphragm a sudden minute displacement of accurately measured magnitude and observing the resulting deflection of a calibrated ballistic galvanometer connected to the receiver coil terminals. 'I'his method proved nsitive and accurate and sumciently rapid to permit a large number of conditions to be examined in a very short time. In the tests, the pole face area. the diaphragm thickness and the length of airgap were systematically varied and a wide range of magnetic materials was investigated. 'Ihe variation of the figure of merit found in one series of tests is illustrated by the closed curves of Fig. 9. Each curve or contour line corresponds to a. fixed value of the gure of merit and the coordinates represent the various combinations of pole area and diaphragm thickness which provide this value.v The intersections of the contour lines with horizontal lines parallel to the 3 thickness axis give successive values of the ligure of merit for different diaphragm thicknesses, and the intersections with vertical lines give the values for different pole face areas.

In the series of tesi-.s resulting in the curve; of

Fig. 9, the magnetic structure used was generally similar in configuration and in materials of construction to that of the receiver shown in Figs. 1 to 6, except that a slightly smaller air-gap of ive thousandths of an inch was used. The maximum value of the figure of merit was somewhat greater than 25 10* in c. g. s. units and was obtained with a. pole face area oi' approximately .030 square inch, and a diaphragm thickness of approximately eleven. thousandths of an inch. The polarizing iluxdensity in the pole-pieces was approximately 6000 lines per square centimeter and in the diaphragm approximately 14,000. An increase of the normal air-gap to six-thousandths of an inch was found to have substantially no effect on the optimum dimensional relationship and resulted only in a slight diminution of the maximum ligure of merit.

In order that the optimum conditions might be Arealized in a corresponding structure employing a permanent magnet, the size and strength of the permanent magnet were so chosen as to produce the same polarizing flux in the pole-pieces and air-gap as was produced by the optimum magnetizing current'in the experimental electromagnet system. This was satisfactorily accomplished by the use of a pair of 35 per cent cobalt-steel magnets, 1.25 inches long and one-sixteenth of a square inch in cross-sectional area, magnetised to a stable condition in accordance with standard practices. Further tests were made upon the permanent magnet structure by superimposing a. variable magnetomotive force upon the magnetic circuit for the purpose of varying the iiux. These showed that the optimum flux condition was substantially realized.

'I'he physical conditions that give rise to the optimum dimensional relationships are of a complex character and do not lend themselves readily to analysis. However, the existence of the grand manmum-*of the figure or mera has been thoroughly established by the tests referred to above.

Moreover, the tests show that the grand maxi-- any changes in the configuration or proportions of the structure or of the value of the polarizing flux. With different magnetic materials, other optimum dimensional f relationships than those indicated by 9 may be found to hold. The tests referred to above have shown that, in gen# eral, the optimum dimensional relationships in-l volve a diaphragm thickness much greater than has heretofore been used. For example, when diaphragme of ordinary magnetic iron are used, the optimum thickness is found to be approxim mately fourteen-thousandths of an inch or greater. The optimum dimensions and the optimum polarizing flux for any particular combination of materials may be determined by following the experimental procedure outlined above.

In the present receiver the diaphragm mate-v rial is characterized by a permeability which re` l tains a high value at high flux densities. It is thought that this characteristic may, at least in part, account for the fact that the optimum diaphragm thickness is less than for other magnetic materials.

From the general formula for the attraction between the faces of an air-gap in a magnetic circuit, it can be shown that the force factor Ci' is expressed approximately in terms of the magnetic fluxes by the equation.

d t fic G11 41r di (9) Where Bo is the polarizing flux density in 'the air-gap and ddl/dz' is the rate of variation ot the flux with current in the receiver winding. The latter quantity is inversely proportional to the reluctance of the magnetic path traversed by the alternating flux produced by oscillating currents in the receiver windings. It follows, then, that vthe figure of merit expressed by Formula 8 is proportional to the ratio of the polarizing flux density to the alternating current reluctance of the magnetic circuit and inversely' proportional to the'diaphragm thickness. These quantities influence each other mutually in a highly complex manner .which prevents a mathematical analysis of the conditions existing at the optimum relationship. The experimental attack outlined above, therefore, represents the best available method for obtaining accurate data.

At the grand maximum, the variation of the figure of merit with the pole face and diaphragm dimensions is not very rapid and a reasonable .60 latitude in the dimensions of these parts may be permitted without noticeable sacrifice of the sensitivity or frequency range. For the magnetic circuit structure already described the diaphragm thickness of .011 inch and the pole face area ci' .031 square inch correspond satisfactorily to the values for the maximum figure of merit when normal air-gap is about .006 to .0065 inch. Under these conditions a figure of merit of the order of 250,000 in c. g. s. units is obtained, which is about twice as great as the value for earlier receivers of similar type.

The relatively great diaphragm thickness for maximum force factor to mass ratio provides' sufllcient rigidity so that the diaphragm may be .75 supported freely at its periphery without danger of being pulled into contact with the pole races by the attraction due to the polarizing flux. The rigidity is also sufficient to ensure the maintenance of a fixed normal air-gap thereby stabilizing the operating characteristics of the system. The absence of edge clamping results in a simpler mode of'ileiiure of the diaphragm under the superimposed force of the speech currents which considerably increases the effective area and the total volume of the air displaced.

While the polepieces have been described hereinabove as L-shaped with side fianges, it is to be understood that considerable variation is permissible in structure. Thus, the side flanges may be omitted, in which case the bar magnets would preferably be in contact with the short leg of the pole-pieces. On the other hand, the pole-pieces might be in the form of an inverted T, in which case the bar magnets would rest on the side members of in the manner shown and would naturally be somewhat shorter. n the case of either of 'these modifications, it would be necessary to have the other dimensions modi iied in order to get the desired force factor to diaphragm thickness ratio. The principle to be followed in making such redesign will be evident from this specification.

A further improvement in the over-all pern formance of the receivers of the invention is obtained by the use of damping arrangements which not only provide the energy dissipation necessary for uniform response, but at the same time substantially diminish the effective mass o' the diaphragm. This result is achieved by the proper coordination of the acoustic and electrical damping whereby the negative reactances in-= troduced by the separate means are of a conl-u plementary character, their sum increasing subm stantially linearly 'with frequency throughoutl the greater portion of the operating range.

Referring to Fig. the acoustic dampingy -elee ments of the system are represented by resistance R1, mass M1 and stiffness S1. lf this combines tion is proportioned so that R1 is approximately equal to the square root of the product M181, its total effective reactance will vary with frequency in the general manner indicated by curve A of Hg. 10. The characteristic features of the variation are a lov:7 vaine of reactance throughout a wide range at lov.T frequencies and a maximum negative value at a relatively high frequency close to the resonance frequency of -mass Mi with stiff ness S1.

Another component or negative reactance is introduced by virtue the coupling to the electrical circuit through the force factor. The im pedance introducen into the mechanical-acoustical portion of the circuit in 'this Way is equal to which is the impedance of anetwork of inverse character to the electrical impedance R, r1, L1, r2, La. The reactance portieri of the introduced impedance has a characteristic variation of the 'type shown by curve E of tig. 10 and is negative in sign. By proper choice of the electrical coefficients, curve B may be made substantially complementary to curve a, giving a total effective reactance of the type indicated by curve C. Ilf'his curve represents a negative reactance which increases substantially linearly with frequency up to or beyond the resonance determined by M1 and S1. The frequency variation of the re-n ctance is of the same character as that of the eil reactance of a physical mass, but the sign of the reactance is reversed. 'Ihe damping elements therefore may be considered as contributing a negative mass which is nearly constant over the operating range and which, being in series with the diaphragm mass, diminishes the total eRective mass of the system. Physically a negative mass would have the propertyof producing an inertia reaction which acts in the direction of the acceleration instead of in opposition thereto, and maybe considered to be present in any system where such a condition exists.

Heretofore in receivers of the magnetic diaphragm type, the force factors have been so low that the requisite values of the transferred impedance to provide the desired characteristics could not be obtained without a very great sacrince of eiliciency. Because of the large force factors obtaining in the receivers of the invention, the desired negative mass effect is obtained 4without substantial loss of efficiency.

In the modified form of the invention shown in Fig. '1, the damping elements take the schematic form shown in Fig. l1, which differs from that of Fig. 8 by the addition of a stiffness S01, directly in series with the mass and resistance. This added stiness is that of the enclosed chamber 41 behind the perforated plate 48. I'he effect of the closed chamber is to diminish the volume and velocity of the air leakage through the perforations, and, hence, to reduce the rate of energy dissipation. To compensate this reduction, the resistance of the damping element must beincreased when the closed chamber is used. Another eifect is to add a stiffness restraint to the diaphragm equal to the stiness of the total enclosed space behind the diaphragm. In Fig. 11. S01 is the stiffness of the enclosed chamber l1, M1' and R1' are, respectively, the mass and resistance of the apertures in damping plate 4I, and S1 is the stiffness of air chamber between the diaphragm and frame Il.

By means of a transformation theorem described by O. J. Zobel, Beil System Technical Join-nal, January 1923, Theory and design of uniform and composite electric wave nlters, appendix III, Transformations A and B, it may be shown that this modified damping can, by suitably proportioning the elements, be made the full equivalent of the combination M1, R1, S1, of Fig, 8, together with a stifl'ness added in series externally to the group. The equivalent schematic is shown in Fig. 12 in which S4 represents the added stiffness.

The curves of Fig. 10 represent the characteristics -of an experimental device of the type shown in Fig. 7 for which the mechanical and acoustic constants had the following values:

R :125 ohms n =700 turns r1 =l8.9 ohms, L1=.00683 henry n =179 ohms, Ia=.00957 henry Ro =300 mechanical ohms Mo =.60 gram So =24 10 dynes per cm.

S1 =49 10 dynes per cm.

S01 =27 10 dynes per cm.

R1' :5600 mechanical ohms S: =19 10 dynes per cm. S: =4.6 10 dynes per cm. M3 :.088 gram R: =130 mechanical ohms G =26 10 c. g. s.

the terms of about 1.4:1 or 1.511.

The corresponding constants of the equivalent damping system, Fig. 12, are:

R1 =2330 mechanical ohms M1=.241 gram s1=a1 1odynespercm- 5 S4 =17.4 10 dynes per cm.

The mass reactance of the diaphragm is represented by straight line D and the effective mass by curve E. the difference between the two being |0 the negative mass reactance indicated by curve C. Dotted straight line F represents the average value of the effective mass reactance. The reduction of the diaphragm mass in this instance is substantially per cent. 'I'he total reactance 15 of the system which is the resultant of the effective mass reactance and the combined reactances of Vthe diaphragm stiffness S0, rear chamber stiffness Si, and front chamber stiffness Sz and Sa, is represented by curve G. The total 30 reactance is zero at 2150 c. p. s., corresponding to the effective resonance frequency of the diaphragm.

From the constants given above, the various significant resonance frequencies of the system u may be determined. The values are as follows: The diaphragm by itself, that is. in the absence of acoustic stiness restraints has a resonance frequency of 1000 c. p. s. With the addition of the acoustic restraints represented by stiffness S4 and 30 the parallel connection of Sz and Sa, the diaphragm resonance is increased to 1400 c. p. s. 'I'he acoustic damping system comprising resistance R1, stiffness S1 and mass M1 is proportioned so that M1 and S1 resonate at about`1850 c. p. s. as The load system represented by ear cavity stiffness Sa, mass M1, and front chamber stiffness Sz is resonant at about 2700 c. p. s. Further. as shown by curve G of Fig. 10, the nal effective resonance of the diaphragm occurs at a frequency 0 in the neighborhood of about 2150 c. p. s., the increase from 1400 c. p. s. representing the effect of the mass reduction due to the combined electrical and acoustical damping.

To secure uniformity of response we have found a it desirable to maintain certain relationships among these resonance frequencies.. Preferably. the three resonance frequencies of the diaphragm and the ear cavity resonance should form approximately ageometric series with a ratio of o Under this condition, the extension of the response range above the resonance frequency of the diaphragm itself is contributed in about equal parts by the acoustic stiffness, the reduction of mass due t0 u damping, and the resonance of the ear cavity; Ihe frequencies 1000, 1400, 2150 and 2700 c. p. s. mentioned, correspond quite closely to the series 1000:1400:1960:2750. In addition, the resonance frequency of the rear damping system should be 00 approximately equal to or greater than the final enective resonance of the diaphragm, in order that the mass reduction may be effective over substantially the whole operating frequency range. In the foregoing example. the values are as 1850 c. p. s. and about 2150 c. p. s., respectively, which is satisfactorily close.

In the device according to Figs. 1 to 6, the absen of the acoustic stiffness S1 tends to reduce the effective resonance frequency of the diaphragm. However, since the mechanical stil!- ness of the diaphragm varies as the cube of the thickness. the reduction may be compensated by a slight increase in the diaphragm thickness. without noticeable sacrifice of the figure of merit. u

assenso l A typical response characteristic for the device of Fig. 'l' is shown in Fig. 13, the ordinates of which represent the acoustic pressure ln the ear cavity in decibels above thestandard threshold value of one bar per' watt of input power. iLe diaphragm resonance and the ear cavity reson-u ance appear as slight undulations at 2l50 c. p. s. .and 2800 c. p. s. At low frequencies, the ull line curve represents the response in completely closed ear cavity corresponding to the reference conditions previouslyl described. The dotted curve shows the eiiect; of normal air leakage between thereceiver cap and the' ear. At a frequency of 300 c. p. s. the loss of sensitivity is 5 decibels, and increases rapidly with decreasing frequency. Since frequencies below 300 c. p. s. are of relatively small importance in speech transmission, the limitation of the response range at the lower range is not of consequence. At higher frequencies the effect of normai air leali-= age is negligible.

As heretofore stated, a single bar magnet as 2l, may be used instead of two of such magm nets as shown in Fig. 6. In case a single magnet is employed, it may be made of an alloy compris-fz ing iron, cobalt and molybdenum in the approximate proportions 72:12:16 and have a cross-secm tional area of about one-sixteenth or" a square inch and a length of about 1.25 inches.

Reference is made to application Serial No. 161,936, led September 1, 1937, a division of this application, wherein certain features oi? the do vices described hereinabove are claimed.

Reference is made also to the application SieYY1 rial.No. 350,741, filed August 3, 1940, or Edward E. Mott, a continuation of this application, wherein certain features of the devices described 'hereinabove are claimed.

What is claimed is:

1. In combination in a telephone receiver, s, diaphragm, and electrical and acoustical means for neutralizing a substantially uniform propor tion of the mass reactance of said diaphragm at all frequencies in the frequency range to be re- 45 produced.

y 2. A telephone receiver comprising a magnetic diaphragm, means for driving said diaphragm,

and means including a highly damped acoustic system associated with said diaphragm for reducm 50 ing the effective mass opposed to the driving force applied to said diaphragm in a substan tially uniform proportion at all the frequencies in substantially vthe entire frequency range to be reproduced.

3. In combination, a telephone receiver com vprislng a diaphragm, acoustic means associated therewith for substantially neutralizing the iect of the mass reactance of the diaphragm over the upper portion of the frequency range to be 60 reproduced, and electrical means for substantially neutralizing the eiliect of the mass react ance of the diaphragm over the lower portion of the. frequency range to be reproduced.

4. The combination of claim 3, characterised 65`by the fact that the electrical reduction of is introduced from the coil windings of the ceiver, which windings are so proportioned to electrical constants as to give the desired neg-aw tive mass effect on the diaphragm.

5. The combination of claim 3, characterized by the fact that the acoustic and the electrical means both introduce damping to hatten out the resonance peaks of the diaphragm vibrations.4

6. The combination of claim 3 in which the combined negative mass eiect of said acoustic 'Frequency range.

telephone receiver comprising a su i teleph receiveras claimed in elailn 7 in which said negative mass eiect producing means comprises an acoustic network and the electrical circuit associated with the receiver.

l0. a telephone receiver comprising a support, a diaphragm thereon, said diaphragm having a resonant frequency in the range of frequencies it is desired to reproduce, means for exerting a constant and a superimposed alternating magu netic force on said diaphragm, said diaphragm being maintained in position on said support sole ly by magnetic attraction, and an acoustic network, coupled to' said diaphragm, said network. having a low positive reactance at frequencies low said resonant ireouency and a negative reu actance thretiugl'iout a range of at least 10Go cycles above said resonant ire-ouency.

li. telephone receiver comprising' a support, a diaphragm thereon, means for exerting a conf: starrt and a superimposed alternating magno ic force o" said diaphrag n, said diaphragm l; maintained in. posi n on said support s by magnetic attraction, an acoustic rieti comprising mass, stiffness, and resi t ance coupled said diaphragm, said resistance being approximately equal to the square root of the product oi? said mass and stiffness.

l2e A telephone receiver comprising a diam phragm having a :ftu'idamental resonance ire quency, an acoustic damping system coupled. to one side oi' ,said diaphragm comprising stliiness, resistance elements, and a load coupling system coupled to the other side ci Said diaphragm comprising stiiness and mass ele ments, .said d "ng and coupling systems o g proportioned 'with respect to the mass and stili ness oi' so that the effective reso nce frequency of the diaphragm is increased substa" tially forty per cent under normal ojo ating conditions by the acoustic stiinesses and. truth-er forty per due to the damping eect of the added acoustic resistance.

A telephone ver in accordance laim in which the said ioad coupling syst reso' or erating conditions at frequency suf tf at y forty per cent than the 'increa d active resonance frequency of the diaphragm.

ifi. telephone receiver having a substantially flat frequency response characteristic up to about 3000 rcycles per second, comprising a diaphragm having a resonant irecuiency oi about i000 cycles secoue ani being peripherally unrestra.l ed, means Ion the front side oi said diaphragm de an acoustic network having a resonant frequency of about 2300 cycles per second, means 'i' on the rear side of said diaphragm deiining an acoustic network having a resonant frequency of about 2000 cycles per second, and means for damping said diaphragm. I

15. A telephone receiver comprising a diaphragm having a resonant frequency in the range of frequencies to be reproduced, means for exerting a constant and a superimposed alternating magnetic force on said diaphragm. a support for said diaphragm, said diaphragm being maintained in position on said support solely by virtue of magnetic attraction, means defining a shallow air chamber on one side of said diaphragm and containing a passage for the egress of sound waves, and an acoustic network including means deiining a shallow air chamber on the other side of said diaphragm and containing an aperture, and acoustic resistance means' disposed across said aperture, said network having a negative reactance increasing with frequency `throughout a range of substantially 1000 vcycles above said resonant frequency. l y

16. A telephone receiver comprising a frame member, a diaphragm supported on said member, said diaphragm having a resonant frequency within the range of frequencies to be reproduced, a plate member on said frame member be- .tween it and the rear surface of the diaphragm to form a shallow air chamber therewith, said chamber being completely closed except for an aperture in said plate member and having subf stantially negligible reactance below said resonant frequency and having negative reactance increasing with frequency throughout a range of frequencies above said resonant frequency. acoustic damping means across said aperture,

" and a cover member on the front side of said diaphragm secured to the frame member but out of contact with said diaphragm and forming an air chamber with the front surface of said diaphragm, said cover member containing a passage therein for the egress of sound waves.

17. A telephone receiver comprising a frame member having a diaphragm supporting surface and a pair of spaced projections on an opposite surface, a circular planar diaphragm resting on said supporting surface, an apertured cover member over said diaphragm and secured to said frame member but out oi' contact with said diaphragm, pole-pieces with horizontal portions bearing against said projections, a pair of bar permanent magnets between said pole-pieces and said frame member, engaging with the horizontal portions of said pole-pieces Aand extending alongside of said projections, a terminal plate bearing against said pole-pieces and supporting a pair of terminal members, means securing said plate and pole-pieces to said projections, a plate member on said frame member between it and the rear surface of the diaphragm to form a shallow air chamber therewith, said plate containit a passage from said chamberand acoustic resistance means disposed across said passage.

18. A telephone receiver comprising a Afrane g member having a shallow recess and an annulla raised portion within the periphery of the frane member, a diaphragm supported at its periph on said raised portion, an apertured cover melber having a peripheral flange portion ont l with the frame member outside of said niet! portion and defining a shallow air chamber with the front side of the diaphragm, a plate mellbl in the recess of said frame member defining a second and relatively more shallow air chants' Il with the rear side of said diaphragm, said plata member containing an aperture leading from .il second chamber and having acoustic daim material thereacross, and means for actua@ the diaphragm including pole-pieces extending n through the plate member and having pole faces in juxtaposition to the rear side of the diaphragm.

19. A telephone receiver comprising a di phragm,means for reducing the effect of the 'il mass reactance 0f said diaphragm throughout a range of frequencies, and means forI reducing the eii'ect of the mass reactanoe of said diaphragm throughout a greater rangeof frequencies. d. first and second means being eifectively substani tially complementary. y

20. A telephone receiver comprising a dhphragm, having a fundamental natural frequency, means for reducing the effect of the mais reactance of said diaphragm throughout a ranle il of frequencies above said natural frequency, sind means for reducing the eil'ect of the man reactance of said diaphragm throughout a range of frequencies extending above and below slid nstural frequency.

21. A telephone receiver in accordance with claim 20 wherein said ilrst and second means are eifectively complementary whereby the resultant reduction thereby of the effect of the mals reactance of said diaphragm is in substantially Imi- 1 form proportion throughout said second range of frequencies.

22. A ltelephone receiver comprising a diaphragm having a fundamental natural frequency, acoustic means associated with said diaphragm for reducing the effect of the mass reactanoe of said diaphragm throughout a range above said natural frequency, said means including a highly damped acoustic network, and additional mem for reducing the effect of the mass reactanee of i said diaphragm throughout a range of frequencies below said natural frequency.

LOUIS A. MORMSON. EDWARD E. MOI'I'. 

